A non-aqueous electrolyte solution type lithium-ion or lithium secondary battery in which a carbon material or lithium metal is used in an anode while a lithium-containing complex oxide is used in a cathode has drawn attention as an electric source for a cellular phone, a notebook computer or the like because it can realize a higher energy density. It is generally known that in such a secondary battery, a film which is called a surface film, a protective film, an SEI or a membrane is formed on an electrode surface. It is also well-known that controlling the surface film is essential for improving electrode performance because the surface film significantly influences a charge/discharge efficiency, a cycle life and safety. Specifically, when using a carbon material as an anode material, its irreversible capacity must be reduced, and in a lithium-metal anode, a charge/discharge efficiency must be reduced while a safety problem due to dendrite formation must be solved.
There have been suggested a variety of procedures for solving the problems. For example, there has been suggested that a membrane layer made of, e.g., lithium fluoride is formed on a surface of lithium metal by a chemical reaction when using lithium metal as the anode material, to minimize formation of dendrites.
Japanese Patent Application No. 1995-302617 has disclosed that a lithium anode is exposed to an electrolyte solution containing hydrofluoric acid for initiating a reaction of the anode with hydrofluoric acid to coat the surface with a lithium fluoride film. Hydrofluoric acid is generated by a reaction of LiPF6 with a small amount of water. On the other hand, a surface film of lithium hydroxide or lithium oxide is formed on the surface of the lithium anode by autoxidation in the air. These react to form a surface film of lithium fluoride on the anode surface. However, since the lithium fluoride film is formed by utilizing a reaction of an electrode interface with a liquid, the surface film may tend to be contaminated with byproducts, leading to an uneven film. Sometimes, a surface film of lithium hydroxide or lithium oxide may not be formed as an even film or in some area lithium metal may be exposed. Such cases may lead to not only an uneven film, but also a safety problem due to a reaction of lithium with water or hydrofluoric acid. An insufficient reaction may lead to a residue of undesired compounds other than fluoride, which may be harmful by causing reduction in anion conductivity. Furthermore, in a process for forming a fluoride layer utilizing a chemical reaction in such an interface, there are limitations to fluorides or electrolyte solutions which can be used. It is, therefore, difficult to form a stable surface film in a good yield.
In Japanese Patent Application No. 1996-250108, a mixed gas of argon and hydrogen fluoride is reacted with an aluminum-lithium alloy to form a surface film of lithium fluoride on an anode surface. However, if a preformed surface film is present on the surface of lithium metal, in particular if a plurality of components are present, the reaction tends to be uneven, which may make it difficult to form an even lithium fluoride film. In such a case, a lithium secondary battery exhibiting satisfactory cycle properties cannot be obtained.
Japanese Patent Application No. 1999-288706 has disclosed that a surface membrane structure comprising a substance with a rock-salt type crystal structure as a main component is formed on a lithium sheet in which an even crystal structure, i.e., a (100) crystal face, is preferentially oriented. It has been thus described that an even precipitation/dissolution reaction, i.e., an even battery charge/discharge can be achieved, resulting in prevention of dendrite precipitation of lithium and improvement in a battery cycle life. A substance used in a surface film preferably comprises a lithium halide, which is preferably comprised of at least one selected from the group consisting of LiCl, LiBr and LiI and its solid solution with LiF. Specifically, for forming a solid solution membrane of at least one selected from the group consisting of LiCl, LiBr and LiI with LiF, a lithium sheet formed by pressing (rolling) in which a (100) crystal face is preferentially oriented is immersed in an electrolyte solution containing at least one selected from group consisting of (1) chlorine molecules or chloride ions, (2) bromine molecules or bromide ions, and (3) iodine molecules or iodide ions and containing fluorine molecules or fluoride ions to form an anode for a non-aqueous electrolyte battery. In this technique, an rolled lithium metal sheet is used. Since the lithium sheet tends to be exposed in the air, a membrane derived from moisture and so on may be formed on its surface, leading to uneven distribution of active sites. It may be, therefore, difficult to form a desired stable surface film. Thus, this technique has not always been effective for adequately preventing dendrite formation.
There has been described a technique that a capacity and a charge/discharge efficiency can be improved when using as an anode a carbon material such as graphite and hard carbon in which lithium ions can be occluded and released.
Japanese Patent Application No. 1993-234583 has suggested an anode where a carbon material is coated with aluminum, whereby reductive decomposition of a solvent molecule in a solvate with a lithium ion on the carbon surface is reduced to minimize reduction in a cycle life. However, since aluminum reacts with a small of moisture, repeated cycles may lead to rapid reduction in a capacity.
Japanese Patent Application No. 1993-275077 has suggested an anode in which a carbon material surface is coated with a film of a lithium-ion conductive solid electrolyte, whereby decomposition of a solvent when using a carbon material can be minimized, so that a lithium ion secondary battery which particularly allows propylene carbonate to be used can be provided. However, change of stress during insertion and removal of lithium ions generates cracks in the solid electrolyte, which may lead to deteriorated properties. Due to unevenness such as crystal defects in the solid electrolyte, an even reaction cannot be achieved in the anode surface, sometimes leading to a reduced cycle life.
Japanese Patent Application No. 2000-3724 has disclosed a secondary battery wherein an anode is made of a graphite-containing material and an electrolyte solution comprises a cyclic and linear carbonate as a main component and 0.1 weight % to 4 weight % both inclusive of 1,3-propanesultone and/or 1,4-butanesultone as a cyclic monosulfonate in the electrolyte solution. It is believed that 1,3-propanesultone or 1,4-butanesultone contributes to formation of a passive membrane on a carbon material surface, which can coat an active and highly crystallized carbon material such as natural or artificial graphite with the passive membrane to prevent decomposition of an electrolyte solution without deterioration of a normal reaction in a battery. Japanese Patent Application No. 2000-133304 and U.S. Pat. No. 6,436,582B1 have described that in addition to a cyclic monosulfonate, a linear disulfonate may be similarly effective. However, using the cyclic monosulfonate in Japanese Patent Application No. 2000-3724 or the linear disulfonate in Japanese Patent Application No. 2000-133304 and U.S. Pat. No. 6,436,582B1, a membrane may be formed mainly on an anode while forming a membrane on, for example, a cathode may be substantially difficult. In addition, Japanese Patent Application No. 1993-44946 and U.S. Pat. No. 4,950,768B1 has disclosed a process for preparing a cyclic sulfonate having two sulfonyl groups. J. Am. Pham. Assoc., Vol. 126, pp. 485-493 (1937) and G. Schroeter, Lieb, Ann, Der Chemie, Vol. 418, pp. 161-257 (1919) and Biol. Aktiv. Soedin., pp. 64-69 (1968), Armyanskii Khimicheskii Zhurnal, 21, pp. 393-396 (1968) has disclosed a process for preparing a linear sulfonate.
In Japanese Patent Application No. 2003-7334, an aromatic compound is added to a solvent for an electrolyte solution for preventing oxidation of the solvent for an electrolyte solution to minimize deterioration in a capacity after long-term repetition of charge/discharge of a secondary battery. It is a technique for preventing solvent decomposition by preferentially decomposing the aromatic compound by oxidation. However, when using the additive, sometimes a cathode surface is not coated, resulting in insufficient improvement in cycle properties.
Japanese Patent Application No. 2003-115324 has described that a nitrogen-containing unsaturated cyclic compound is added to an electrolyte solution to improve cycle properties when using a high-voltage cathode. However, while the nitrogen-containing unsaturated cyclic compound can improve a charge/discharge efficiency in an anode, it cannot improve a charge/discharge efficiency in a cathode.